Illustration of dendritic cells (white spheres) and red blood cells (erythrocytes, red discs) in a blood vessel. Lymphocytes continually recirculate between the blood and the tissues via the lymphatic system and form part of the immune system. Dendritic cells are antigen-presenting cells (APCs), that is, they present pathogens or foreign molecules (antigens) to other cells of the immune system to be eliminated.